/// <summary>
/// Adds the listener. If it is not present, executes the visitor operation on the added listener.
/// </summary>
public virtual void addListener(T listener, Visitor <T> visitor)
{
lock (this)
{
bool isNew = listeners.Add(listener);
if (isNew)
{
eventThread.queue(() =>
{
visitor.visit(listener);
});
}
}
}
